1. Field of the Invention
This invention relates to a method of increasing the abrasion resistance of a chromium-bearing heat hardenable stainless steel while retaining good corrosion resistance and ability to be readily converted to wrought products by hot and cold working with conventional steel mill equipment. Steel treated by the method of the invention is martensitic in the heat hardened condition. The invention further relates to a steel of critical composition which has particular utility for fabrication into bearings, ball joints, tire studs, cutlery, materials processing equipment such as mining and ore processing machinery, and similar products the ultimate use wherein the above combination of properties is needed.
2. Description of the Prior Art
Currently available alloys capable of withstanding high stress, abrasive conditions are produced as castings only and are not amenable to production in wrought form. Among such prior art iron-base alloys are chromium-molybdenum white cast iron (analyzing about 3.2% carbon, about 0.6% silicon, about 15.0% chromium, about 3.0% molybdenum, and balance iron), and high chromium white cast iron (analyzing about 2.7% carbon, about 0.65% silicon, about 27.0% chromium and balance iron). Other such alloys are tool steels, e.g. AISI Type D-2(1.50--1.60% carbon, 0.30-0.45% silicon, 11.50-12.50% chromium, 0.75-0.85% molybdenum, 0.70-0.90% vandium, and balance iron), and AISI Type D-4 (2.0-2.30% carbon, 0.20-0.45% silicon, 11.50-12.50% chromium, 0.70-0.90% molybdenum, 0.30-0.50% vandium and balance iron).
Prior art martensitic stainless steels classified as wrought steels, such as AISI Types 440 A, B and C, actually can be hot worked and cold worked in standard mill equipment only with great difficulty. Moreover, these steels, which contain up to about 1.2% carbon, are deficient in abrasion resistance under very high stress, abrasive conditions.
U.S. Pat. No. 3,692,515 issued Sept. 19, 1972 to S.G. Fletcher et al, discloses a steel alleged to have improved abrasion resistance, forgeability and workability consisting essentially of about 1% to about 4.25% carbon, about 1.5% maximum silicon, about 1.5% maximum manganese, about 10% to about 15% chromium, less than 2% molybdenum, about 0.5% to about 5% titanium, less than 3% tungsten, less than 3% nickel, less than 5% cobalt, less than 5% vandium, up to 0.25% sulfur, and balance iron with residual impurities. A preferred composition contains 2.9% carbon, 0.4% silicon, 0.4% manganese, 12.5% chromium, 1.1% molybdenum, 3% titanium, and balance substantially iron with residual impurities. It is stated that carbon is added in excess of that necessary to give a desired hardenability and that such excess carbon is combined with titanium in a weight ratio of 4:1 and vanadium in a weight ratio of 4.2 (V-1):1. The cast alloy is reduced in cross sectional area by at least 5% by working, and heat treated by austenitizing at 1600.degree. to 1950.degree. F and tempering at 900.degree. to 950.degree. F.
The maximum austenitizing temperature of 1950.degree. F disclosed in the Fletcher patent limits the amount of dissolved carbon to about 0.7% to 0.8% maximum. If no vanadium is present, the excess carbon content in the preferred practice would be Ti/4, or 3/4 (the preferred titanium content being 3%), i.e. 0.75%. Thus the total carbon content should be 1.45% to 1.55%. Since the excess carbon cannot all be dissolved and since the amount of titanium is insufficient to combine with all the excess carbon, that portion of the carbon not in solution and not in the form of titanium carbides would appear as ledeburitic carbides of iron, chromium, and such optional elements as vanadium, molybdenum and tungsten.
The limited disclosure of the Fletcher patent regarding heat treatment gives no indication of the microstructure of the tempered product and would apparently result in the presence of retained austenite.
There is thus a real need for a method of increasing the resistance to erosion by mechanical and/or mechanical-chemical abrasion in a heat hardenable stainless steel, which also exhibits ease of manufacture and fabrication into articles of ultimate use, and good corrosion resistance.